1. Field of the Invention
The invention relates to an improved piezoelectric element, for example for a piezoelectric actuator for actuating a mechanical component, such as a valve or the like.
2. Description of the Prior Art
It is well known that by utilizing what is known as the piezoelectric effect, a piezoelectric element can be constructed from a material having a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element ensues, which as a function of the crystalline structure and of the regions where the electrical voltage is applied takes the form of compression or tension in a predeterminable direction. Because of this extremely fast and precisely regulatable effect, piezoelectric actuators of this kind can be provided for constructing final control elements, for instance for driving switching valves in fuel injection systems in motor vehicles. In that case, the voltage- or charge-controlled deflection of the piezoelectric actuator is utilized for positioning a control valve, which in turn regulates the stroke of a nozzle needle. A major advantage of piezoelectric actuators is the achievement of precise and very fast deflections with high forces.
Since the requisite electrical field intensities for actuating the piezoelectric actuator are in the range of several kV/mm, and as a rule moderate electrical voltages are desired for triggering purposes, the construction of this piezoelectric actuator can be done in multiple layers approximately 100 μm thick each (multilayer actuators). To that end, inner electrodes are present between the respective layers and are applied for instance by a printing process, and outer electrodes are present by way of which the electrical voltage is applied.
Because the ceramic layers here are connected electrically parallel, the requisite control voltage drops, compared to monolithic piezoelectric actuators of the same length and with the same number of layers. A typical method for producing such layers is sheet casting, in which the individual layers are metallized and stacked one above the other.
From European Patent Disclosure EP 0 844 678 A1, one such piezoelectric actuator is known in which two outer electrodes of different polarity are present, applied to respective opposed sides of the piezoelectric actuator block. Upon contacting of the inner electrodes with the lateral outer electrodes, the contacting changing from one layer to the next, the respective contacting takes place in the region where in the respective adjacent layer, no inner electrode is extended to the outside.
The layers having the inner electrodes function among one another as capacitors, so that to assure the electrical discharge of the total capacitance, a shunting resistor is as a rule connected parallel. For instance, this can be done with a wire resistor in the region of a plug of a connecting device. However, in that case the production process is relatively complicated, since the resistor must additionally be soldered and also requires additional space. Moreover, care must be taken to assure that this arrangement also withstand jarring stresses, which makes the construction more difficult.
At the head and foot part of these known piezoelectric actuators, the inner electrodes are as a rule absent, since on the one hand a certain insulation distance from the end faces is necessary to prevent short circuits to the outside, and on the other, passive zones are utilized for electrically connecting the outer electrodes.